The performance of susceptibility tests for mycobacteria other than Mycobacteria tuberculosis is time-consuming, has not been well standardized, and has not been correlated with clinical results. The Dawn instrument (Wyatt Technology Corporation, Santa Barbara, CA) is a laser light-scattering instrument which sensitively detects changes in bacterial number and shape. This project will investigate the utility of this instrument for assessing the susceptibility of mycobacteria to antimycobacterial drugs by assessing the changes produced by the drugs as measured by the instrument. Results so far indicate that the instrument may be Useful for relatively determining drug susceptibility in mycobacteria, although some changes in instrument design would be necessary to optimize the usefulness Of the instrument for the diagnostic laboratory.